1. Field of the Invention
The invention relatives to a current adjustment apparatus, more particularly, to a Pulse Width Modulation (PWM) current adjustment apparatus.
2. Description of The Related Art
A current adjustment apparatus is a common component in a driver circuit for a light emitting diode (LED). FIG. 4 represents a conventional current adjustment apparatus working in PWM (Pulse Width Modulation) mode. This current adjustment apparatus comprises a sawtooth wave generator 1, a comparator 2, a field effect transistor 3 (FET), a power supply 7, and current limiting resistors 4, 5. One input of the comparator 2 is connected to an output of the sawtooth wave generator 1, and the other input of the comparator 2 is connected to a modulation voltage source 6. The output of the comparator 2 is connected to a gate terminal of the FET 3. The current limiting resistor 4 is connected between the power supply 7 and a source terminal of the FET 3, and the resistor 5 is connected to a drain terminal of the FET 3.
Referring to FIG. 5, Vi1 is a modulation voltage signal provided by the modulation voltage source 6, Vr1 is a sawtooth wave signal provided by the sawtooth wave generator 1, and Vo1 is a voltage applied to the gate terminal of the FET 3. In use, the sawtooth wave signal Vr1 is compared with the modulation voltage signal Vi1 in the comparator 2. The comparator 2 outputs a positive level signal when the modulation voltage signal Vi1 is higher than the sawtooth wave signal Vr1, and the comparator 2 outputs a zero level signal when the modulation voltage signal Vi1 is lower than the sawtooth wave signal Vr1. The output level signal Vo1 is applied to the gate terminal of the FET 3, and the FET 3 outputs a current Io1 to a load (not shown) through the resistor 5.
However, the conventional sawtooth wave generator 1 suffers from some disadvantages. Referring to FIG. 6, a circuit of the conventional sawtooth wave generator 1 is complex since it has two integral paths, i.e. a forward one D1-R3-C and a backward one C-R4-D2. Furthermore, considering a Fourier series expansion of a sawtooth wave signal:V=(2/π)Vm[sin wt−(½)sin 2 wt+(⅓)sin 3 wt . . . +(−1)n−1/n sin (n) wt+ . . . ],the Fourier series expansion reveals that a sawtooth wave signal V comprises both even harmonics and odd harmonics, and includes a considerable percentage of high frequency harmonics, which induces high frequency noise in the system and makes it difficult to built a high frequency sawtooth wave generator, and directly effects the stability of output current. In additional, to adjust the output driving current Io1 in PWM mode, a frequency of the sawtooth wave signal Vr1 must be 10 times higher than that of the modulation voltage signal Vi1 in order to reduce unexpected harmonics in the output. A new current adjustment apparatus which solves these problems is desired.